Design of support structures for offshore wind turbines - Wind Power Generation and Wind Turbine Design - page 569

To avoid scour, monopiles are provided with scour protection . Protection against
scour is usually done by placing a “fi lter” layer of small stones around the pile. On
top of that a layer of larger stones is positioned. The small stones keep the sand
around the pile in place and the large stones keep the fi lter layer in place.
The relatively simple production and installation, together with the large range
of exploitable water depths, have made the monopile the most widely used support
structure concept. Its popularity has led the monopile to be developed for increas-
ingly deeper waters. Monopiles are therefore likely to remain the most popular
foundation type in the near future.

3.2.2 Tripod
A tripod foundation is a structure with three legs which diverge from a single node
to their respective positions on the seabed. A foundation pile is driven into the
ground at the base of each leg of the tripod section. On top of the tripod section,
the turbine tower is placed. The procedure is visualised in Fig. 15.
Complications with production and installation make it relatively expensive.
The main transition node where the three legs meet the central column is sensitive
to fatigue. Stiffness benefi ts are only interesting in large water depths, but then the
base becomes restrictively large.
The conventional installation method is to load several tripods onto a barge
which is towed to the offshore site as depicted in Fig. 16. At a predetermined loca-
tion, a structure is lifted of the barge, using a large crane (on the barge). Simultane-
ously, a smaller crane guides the tripod to its fi nal position. The loads on the tripod

Lifting and
landing of tripod

Foundation piles Turbine Tower Nacelle Rotor blades

Figure 15: Installation sequence of main components for a tripod foundation.

Figure 16: Tripods on a barge, on their way to the Alpha Ventus wind farm.